It is always difficult to operate on an assembled sprung balance assembly to adjust the oscillation frequency of a sprung balance, because of the risks of modifying the balance spring. The operation is even more difficult when envisaged in a timepiece movement that is already assembled, because of constraints for the protection of other components, and the cleanliness of the movement.
FR Patent No 2179744 A1 in the name of Far discloses a method of poising and adjusting the oscillation frequency of a sprung balance, by removing material from the balance felloe using a cutting tool, in correlation with measuring and drive means, without any particular protection of the balance spring.
CH Patent No 532284 A in the name of Suwa Seikosha KK discloses a rotor poising device, and re-machining of the balance, with the balance spring secured to the collet. This re-machining is performed with a drill, which requires stopping and holding the balance to carry out the machining operation.
CH Patent No 704211 A2 in the name of Swatch Group R&D concerns the protection of the balance spring against electromagnetic radiation, and this document indicates a rule of proportion between the diameter of the balance and the diameter of the outer coil of the balance spring, with no mention of the terminal curve or the attachment to the balance spring stud, or any contact with an index.
US Patent No 2012/157743 A1 in the name of Silva Regio-Roger Dubuis SA discloses the assembly of a balance spring taken from a series of springs, on a balance via a collet affixed to the balance, said collet being selected from among a set of collets which have points of attachment to the balance spring that may be closer or farther from the staff.
EP Patent No 2315082 A2 in the name of Audemars Piguet & Co discloses a regulating member including two coaxial balances.